Why the change of "Stance"?

Can anyone explain: How come Tyr has an ult while Hel and Ullr doesn't? What's the difference with Tyr being a warrior with an ult while Hel as a mage and Ullr as a hunter with no ult?

There's quite a huge difference between being able to switch stances every other second and being able to switch stances every ten or so seconds. The loss of a true ultimate is the trade-off for not being stuck in a stance for an extended period of time.

Maybe cause his stances don't change his playstyle or something?

I mean in both his stances his abilities still do damage and are still used offensively, only with different effects that don't alter too much. An example is his Cleave, which in both stances is still a damaging ability, only in Defense stance it has the added benefit of a heal.

Hel's abilites change drastically when switching her stances. Instead of having the debuff you instead get a cleanse, and instead of your big AoE ability that deals ****loads of damage you get a massive heal.

But thats just my thought, so take them with a grain of salt.

My guess would be that when they made Hel, they thought continuous stance change was maybe too op. So when Tyr came out, I think they made him an experimental character. I guess their goal was to see: "is it better to have switch stance character with a damaging ultimate?" Well, as everyone probably knows, Tyr with CDR IS A MERCILESS KILLING MACHINE... of death. Used your ult on damage stance, BOOM huge damage, then a 1,2,3,1,2,3 combo to finish you off. Now the CDR part comes in, "hey you're a tank? No prob!!!! I'll just spam my stance change like a little b**** because I have CDR!!!!!! Don't worry since I'm brusing I got BoV so I don't run out of mana too quickly! Pssst more CDR" so I guess they could't change his abilities since players are already familiarized with them. When Ullr came around they didn't want him having a good damage initiator ult that would make the stance change an ability that would be pretty impossible to cool down with CDR.